Semiconductor lasers have many advantages, such as small, inexpensive, low power consumption and long service life, and they have been widespread in a wide variety of fields, such as light sources for optical recording, light sources for communication, laser displays, laser printers, and laser pointers. Meanwhile, in the field of laser processing or laser machining, lasers having an optical output power exceeding at least 1 W are required. However, semiconductor lasers currently in actual use do not achieve this output due to the reasons later described. Thus, currently in the laser machining field, gas lasers such as carbon dioxide gas lasers are used instead of semiconductor lasers.
The reason why the optical output powers of semiconductor lasers currently in use are low is as follows. To increase the optical output power of the semiconductor laser, it is preferable that the laser beam emitted from the laser element has a large sectional area (emission area). On the other hand, to increase the machining accuracy, it is preferable that the laser beam to be spotted onto a workpiece has a small sectional area (spot area).
Consequently, ideally speaking, it is desirable that the laser beam emitted from a laser source reach the workpiece without broadening. However, in the semiconductor laser, as the emission area increases, the spread angle of the laser beam increases and the wavefront distortion of laser light occurs. If the wavefront distortion of the laser light occurs, it is difficult to obtain a small spot area even when light is focused using an optical system. It is thus difficult for the semiconductor lasers currently in use to achieve the optical output power of 1 W or higher while achieving a small spread angle.
Recently, Noda and Liang among the present inventors have developed a two-dimensional photonic crystal surface emitting laser having the following characteristics: an optical output power of 1.5 W and a beam spread angle of 3° or less (Non-Patent Literatures 1 and 2). The two-dimensional photonic crystal surface emitting laser includes: a two-dimensional photonic crystal including a plate-shaped base member and modified refractive index regions where the modified refractive index regions have a refractive index different from that of the plate-shaped base member and periodically arranged in this member; and an active layer. In the two-dimensional photonic crystal surface emitting laser, when electric current is injected into the active layer, only light having a predetermined wavelength corresponding to the periodicity of modified refractive index regions is amplified among light generated in the active layer, and is caused laser oscillation, whereby a laser beam is emitted in the direction perpendicular to the two-dimensional photonic crystal. The two-dimensional photonic crystal surface emitting laser emits light (surface emission light) from a certain area in the two-dimensional photonic crystal. Consequently, this laser has a larger emission area than that of an end surface emission semiconductor laser, which can enhance the output power and reduce the spread angle. Conventionally, various two-dimensional photonic crystals including modified refractive index regions with different planer shapes (circle, regular triangle, etc.), arrangements (triangular lattice, square lattice, etc.) or other parameters have been known. In the two-dimensional photonic crystal surface emitting lasers described in Non-Patent Literatures 1 and 2, the modified refractive index regions having a planar shape of right triangle are arranged on the lattice points of a square lattice parallel to the orthogonal sides of the modified refractive index region, whereby the optical output power is increased from the conventional two-dimensional photonic crystal surface emitting lasers.